1. Field of the Invention
The invention relates to sensing devices, and aspects thereof, which exploit the unique properties of discotic liquid crystals (DLC).
2. Description of the Related Art
DLCs essentially comprise an aromatic core surrounded by several aliphatic side chains. Typically, a number of aromatic cores are positioned in an aligned, stacked fashion so as to provide for a columnar arrangement. Further, the columns tend to organise into a two dimensional superlattice providing, for example, a hexagonal structure, hexa-alkoxytriphenylenes are well known representatives of this structure of DLCs.
DLCs attracted considerable attention when it was discovered that the columnar phase structure of DLCs was suitable for fast transport of charge carriers. More specifically, it was established that the orientation of the columns directed the flow of this charge because the charge essentially travelled along each column and was further insulated from adjacent columns by the aliphatic side chains attached to the aromatic cores. As a result of this knowledge the use of DLCs in the electronics industry has grown and it is of note that it is the transfer of electric charge along the axis of the columnar DLCs that has been exploited.
However we disclose in this Patent Application a new property of DLCs and a novel way in which this new property can be exploited.
Fluid sensing devices, and in particular gas sensors, are becoming increasingly important for monitoring industrial environments. In particular, they are desirable for use in the chemical industry where the detection of leaks and in particular the detection of leaks of a hazardous nature must be continually monitored. As a result of this various gas sensors have been developed. The most sophisticated is based on polymer technology and essentially involves the interaction of a gaseous molecule with a given polymer and the recording of a response as a result thereof. More specifically, the sensor uses electrically conducting organic polymers based on heterocyclic molecules such as pyrrole. Each polymer is a different functional unit that displays reversible changes in conductivity when it is exposed to polar volatile chemicals. Usually, an array of polymers are provided and the interaction of a gas molecule, or a cocktail of gas molecules, when exposed to each of said polymers in said array is monitored and the subsequent response, or fingerprint, is recorded. Thereafter when the same sensor array is exposed to the same gas, or combination of gases, the same response, or fingerprint, is noted. In this way gas sensors can be xe2x80x9ctrainedxe2x80x9d to detect different gases, or combinations of gases, and so be programmed to monitor different, environments for leaks.
However, the aforementioned sophisticated technology is not sufficiently sensitive to detect all kinds of gases and in particular it is not able to detect all organics. Notably, it cannot detect non-polar hydrophobic organics such as benzene and as a result of this its application is not universal.
It is therefore an object of the invention to provide a fluid sensor and in particular a gas sensor which detects a wide range of gases and in particular organic based gases.
It is yet a further object of the invention to provide a sensor and in particular a gas sensor based on the use of DLCs.
It is yet a further object of tie invention to provide a sensor array comprising at least one and preferably a plurality of DLCs and ideally a plurality of differing DLCs.
It is yet a further object of the invention to exploit a new use of DLCs, that being for the detection of fluids and in particular gases and more particularly further still non-polar organic based gases.
It is yet a further object of the invention to provide a sensor that operates in real time.
According to a first aspect of the invention there is therefore provided a fluid sensing device comprising a substrate on which there is provided at least one type of discotic liquid crystal and further wherein there is also provided contact means adapted so as to measure the flow of electric charge through the upper part of the said discotic liquid crystal.
It will therefore be apparent from the above, to those skilled in the art, that we have identified a novel property of DLCs, that being the ability to conduct a surface charge, that is a charge generally perpendicular to the axis of the columns forming the DLCs. Moreover, we have also discovered that this surface conductivity can be affected by fluids and in particular gases such as organic based gases. It therefore follows that the surface conductivity can be used to monitor the levels of, or existence of, fluids and in particular gases in a given environment.
Advantageously the effect on surface conductivity is very fast and so the device is able to operate in real time.
In a preferred embodiment of the invention a plurality of discotic liquid crystals are provided and ideally on a single substrate. Preferably the plurality of discotic liquid crystals are positioned on such substrate so as to provide for an array.
In the preferred embodiment of the invention the response of different discotic liquid crystals to different fluids or gases can be determined, and where an array is provided a given gaseous molecule, or combination of gaseous molecules, can interact with the said array so as to provide a given, typically unique, response. This response can then be recorded and used for future analysis of gases, either identical to the original gas, or gases, or differing therefrom.
Ideally, the said device is also provided with an information storage and retrieval facility whereby data relating to different fluids and in particular gases can be stored and accessed so that analysis of gases or environments can be facilitated.
In yet a further preferred embodiment of the invention said at least one discotic liquid crystal is 2, 3, 6, 7, 10, 11 hexa-hexyloxytriphenylene (HAT6).
More preferably still said discotic liquid crystal comprises at least one such crystal shown in table 1 and exemplified in FIGS. 12 and 13. Ideally said discotic liquid crystal comprises a plurality of the discotic liquid crystal shown in table 1 and exemplified in FIGS. 12 and 13, and ideally each gas sensing device comprises a selected combination of said discotic liquid crystals which combination is selected having regard to the purpose of the sensor. Therefore, for example, where given discotic liquid crystals are shown to be particularly sensitive to a given gas, or combination of gases, then these discotic liquid crystals will be employed in sensors used to detect gases, or combinations of gases, for which they have exemplified favourable sensitivity.
Although the invention has been described with reference to the discotic liquid crystals shown in table 1 and exemplified in FIGS. 12 and 13 it will be understood by those skilled in the art that the invention is not to be limited by the examples of discotic liquid crystals specified in this application, rather the invention lies in the realisation that discotic liquid crystals can be used, because of their surface conductivity, to detect fluids and in particular gases. Thus the number and nature of discotic liquid crystals that can be used in the invention are limitless, as is their combination, selective or otherwise, typically for use in an array.
Surprisingly, we have found that our device responds to different gases whatever the thickness of the DLC layer and we consider this to be because the upper conducting surface remains constant. Indeed, we have found that the thinner the layer the lower the surface resistance and so the greater the conductivity. As a result of this we prefer to use sensors that comprise a relatively thin film of at least one DLC, for example, the said film is typically less than one micrometre and more preferably still less than 0.5 of a micrometer and ideally in the order of 0.1 micrometers.
According to a second aspect of the invention there is provided a sensor array, for use in a fluid sensor, comprising at least one type of discotic liquid crystal.
In a preferred embodiment of the second aspect of the invention a plurality of different discotic liquid crystals are used. More preferably still means are provided to measure surface current flow across each type of discotic liquid crystal.
According to a yet third aspect of the invention there is provided the use of at least one discotic liquid crystal for use in detecting a fluid and in particular a gas, especially an organic based gas such as a non-polar gas.
Whilst we would not intend for the invention to be limited by the following explanation, we consider that discotic liquid crystals function as fluid, and in particular gas, sensors because of the ability of the aliphatic side chains to interact with said gases and so affect the organisation at the surface of the discotic liquid crystal structure. The enhancement or depression of surface organisation thus affects the ability of the surface to conduct charge and we speculate, that in this way, a given fluid interacts with the surface of a discotic liquid crystal to affect surface organisation and thus charge flow therethrough.
We believe that DLCs will be especially useful for sensing non-polar hydrophobic volatile chemicals in vapour sensing instruments usually referred to as xe2x80x9cthe electronic nosexe2x80x9d. These instruments, which detect smell levels down to parts per billion normally use an array of 8, 16 or 32 separate gas sensors. It is thought that DLCs may be used in such sensors and exploited because of the ability of DLCs to detect non-polar hydrophobic gases.
An embodiment of the invention will now be described by way of example only with reference to the following Figures wherein: